USPTO Patents Application 09839759

PATENT
1085-2

ETHANE EXTRACTION PROCESS FOR A HYDROCARBON GAS STREAM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to United States Provisional Patent

Application Serial No. 60/202,401, filed May 8, 2000, the contents of which are incorporated by
reference herein.

FIELD OF THE INVENTION

[0002] The present invention relates generally to a method for recovering ethane from a

hydrocarbon gas stream. More particularly, the present invention relates to an improved method
having turboexpansion of a methane-rich stream to provide refrigeration in an ethane extraction
process which is used to recover an ethane product.

BACKGROUND ON THE INVENTION

[0003] The production and consumption of ethane is rising because it is a valuable

petrochemical feedstock for ethylene manufacture. Furthermore, highly pure methane and
propane are also valuable products of an ethane extraction plant.

[0004] Figure 1 is simplified schematic of a typical ethane extraction plant. A

hydrocarbon feed, containing methane, ethane, propane and possibly heavier hydrocarbons, is
fed to a demethanizer column. The overhead of the demethanizer column is typically a methane-
rich stream. The bottom of the demethanizer column is fed to a de-ethanizer column. An
ethane-rich stream is recovered overhead and a propane-rich stream is recovered from the bottom
of the de-ethanizer column.

[0005] The hydrocarbon feed is often cooled through a refrigeration section (not shown)

prior to entering the demethanizer column. Refrigeration is a usefiil technique for achieving low

temperatures necessary for the separation of methane from the other hydrocarbon constituents in
the hydrocarbon feed.

[0006] Several methods currently exist for recovering methane, ethane and propane from

hydrocarbon gas streams. Some typical examples of isolating and extracting ethane and or
propane are disclosed in U.S. Patent Nos. 4,278,457; 4,597,788; 4,689,063; 4,851,020;
4,869,740; 5,275,005; 5,568,737; 5,588,306; 5,890,377; and 5,953,935.

[0007] Existing ethane extraction plants use (i) propane refrigerant as the cooling means

for ethane separation or (ii) a combination of turboexpansion and Joule-Thompson expansion of
the hydrocarbon feed. The use of turboexpansion on a gas feed to an ethane extraction plant is
possible when the gas stream is lean, i.e., having a low concentration of hydrocarbons heavier
than methane. When a gas stream is rich in ethane or propane, these hydrocarbons would
typically condense into liquids during cooling, resulting in insufficient cooling at the
turboexpansion facilities associated with the gas feed.

[0008] For a gas stream rich in ethane or propane, however, a propane refrigeration

system is commonly used to cool the gas feed stream. Propane refrigeration, however, often
becomes inefficient with lean feeds.

[0009] A cold methane stream has been used to partially cool the gas feed to an ethane

extraction plant. A process disclosed in U.S. Patent No. 4,687,499 used a cool overhead
methane stream from a demethanizer column as a cooling source for a natural gas feed. The cool
demethanizer overhead methane stream cooled the natural gas feed in a heat exchanger. The
methane stream was then compressed before being recovered as a methane product. This
process, however, is limited in overall efficiency because it does not fully utilize the cooling
potential of the methane stream.

[0010] There is a need for an ethane extraction process capable of efficiently and

economically extracting ethane from both lean and rich gas feeds.

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SUMMARY OF THE INVENTION

[0011] The present invention is a process for recovering ethane from a hydrocarbon gas

feed stream containing methane, ethane, propane and possibly heavier hydrocarbons. Cooling of
the feed stream is accomplished in an exchanger, such as a cryogenic heat exchanger.
Turboexpansion of a methane-rich stream provides a cooling source for the cryogenic heat
exchanger to cool the hydrocarbon gas feed.

[0012] The cooled hydrocarbon gas feed is separated into a methane-rich stream and an

ethane/propane-rich stream in a demethanizer column. An ethane-rich stream is recovered from
a de-ethanizer column.

^3 [0013] The turboexpansion of a methane-rich streams permits the processing of lean

ID and/or rich hydrocarbon feed streams while avoiding undesirable condensation that may lower
cooling capability of the refrigeration system, such as the cryogenic heat exchanger.

iO [0014] In one aspect, the present invention is a method for recovering ethane from a

hydrocarbon gas stream having methane, ethane and propane which includes the steps of (i)
=C providing the hydrocarbon gas stream; (ii) cooling the hydrocarbon gas stream by refrigeration to
n form a cooled hydrocarbon gas stream; (iii) separating the cooled hydrocarbon gas stream into a
l"^ methane-rich stream and an ethane/propane-rich stream; (iv) expanding the methane-rich stream
from a high pressure to a lower pressure to lower the temperature of the methane-rich stream
which then provides a cooling source for the refrigeration of the hydrocarbon gas stream; (v)
separating the ethane/propane-rich stream into an ethane-rich stream and a propane-rich stream;
and (vi) recovering methane-rich, ethane-rich and propane-rich product streams.

[0015] Desirably the step of expanding the methane-rich stream includes the steps of (i)

compressing the methane-rich stream; (ii) cooling the compressed methane-rich stream; and (iii)
turboexpanding this cooled and compressed methane-rich stream.

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BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is simplified flow chart for recovering methane, ethane and propane by

distillation of a light hydrocarbon gas feed.

[0017] Figure 2 is a flow chart of the process of the present invention for recovering

methane, ethane and propane from a light hydrocarbon stream by distillation and turboexpansion
of a methane-rich stream which is used to cool the refrigeration system of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0018] The process of the present invention is capable of producing an ethane-rich stream

with the following typical, but non-limiting, characteristics:

Methane: 0.5% molar maximum,

Ethane: 96.5% molar minimum, and

Propane: 3% molar maximum.

[0019] The feed for the process of the present invention is a hydrocarbon mixture gas at

about 25°C and about 70 kg/cm^abs. The process of the present invention can recover
hydrocarbon products from lean, normal and rich hydrocarbon mixtures. Typical properties of
these feeds are shown below in Table 1.

Table I

Hydrocarbon Feed Compositions

Composition

Nitrogen
CO2

Methane (Cj)
Ethane (C2)
Propane (C^)
C4+ Hydrocarbons

Lean

Normal

Rich

mole fraction

0.0010

0.0010

0.0010

0.0760

0.0760

0.0748

0.7338

0.6328

0.5316

0.1762

0.2700

0.3573

0.0130

0.0200

0.0350

0.0000

0.0002

0.0003

4

[0020] The process of the present invention is not limited to the processing of the feeds

described above in Table I, but the inventive process can handle a wide range of feed
compositions. The above-described values are cited as illustrations of hovs^ the invention process
performs under different lean and rich hydrocarbon compositions.

[0021] Carbon dioxide and w^ater should be removed from the hydrocarbon feed to avoid

solid formation (due to freezing of the components) in cold sections of the plant. The col d
sections can include refrigeration sections, distillation columns, heat exchangers and other
associated equipment operating at conditions that would lead to the freezing of carbon dioxide or
water. Carbon dioxide is typically removed from the feed by an amine absorption unit. Water is
often removed from the feed by a molecular sieve unit. Desirably the levels of carbon dioxide
and water in the feed gas after these treatments are less than about 0.02% and 0.0001% molar,
respectively.

[0022] Figure 2 is a schematic flow diagram of the process of the present invention. The

hydrocarbon gas feed, which is typically at about 25'^C and about 70 kg/cm^abs, is first
sweetened and dehydrated by a conventional amine process (not shown) and by a molecular
sieve unit (not shown). The hydrocarbon gas feed is then typically precooled with cooling water
at a heat exchanger (not shown).

[0023] The precooled hydrocarbon gas feed stream 1 is fiirther cooled at gas-gas

economizer 233 to provide a feed stream 3. The feed stream 3 is then divided in two parallel
streams 5, 13. The parallel feed stream 5 is cooled in exchanger 231 to about 10 °C by heat
exchange with ethane-rich streamf91^o yield a cooled feed stream 7. The ethane-rich stream 91
is at about -6°C coming out from the top of the de-ethanizer column 203 via heat exchanger 229
and separator 207. The cooled feed stream 7 is fiirther cooled to about -3°C to provide a further
cooled feed stream 9 by supplying heat to one side reboiler 211 of the demethanizer column 201 .
This fiirther cooled feed stream 9 is passed through a cryogenic heat exchanger 223, which is
desirably a cryogenic brazed alimiinum heat exchanger. The further cooled feed stream 9 is
partially condensed in the cryogenic heat exchanger 223 to yield a partially condensed feed

stream 1 1 . The temperature of the partially condensed feed stream 1 1 is at about -26°C. The
partially condensed feed stream 1 1 is then forwarded to separator 205.

[0024] The parallel feed stream 13 is cooled in exchanger 209 to about -6°C by heat

exchange with a substantially liquid stream 81 coming from the bottom of the demethanizer
column 201 . The cooled feed stream 1 5 from exchanger 209 is further cooled to about -26°C by
supplying heat to one side of reboiler 213 of the demethanizer column 201 to yield a further
cooled feed stream 17. The further cooled feed stream 17 which, is partially condensed, is fed to
a separator 205 together with the other partially condensed feed stream 1 1 coming from the
cryogenic heat exchanger 223.

[0025] In the separator 205, vapor is separated from condensed liquid. The vapor 19

from the separator 205 is passed through the cryogenic heat exchanger 223 where it is
substantially condensed at a temperature of about -89*'C. The substantially condensed feed
stream 21 from the cryogenic heat exchanger 223 is then expanded through an expansion valve
239 to the operating pressure of the demethanizer column 201 to yield demethanizer feed stream
23.

[0026] During expansion, the substantially condensed feed stream 21 is cooled to

approximately -94°C to yield the demethanizer feed stream 23. The demethanizer feed stream 23
is fed to a location near the top of the demethanizer column 201 . The reduction of temperature
as most gases are expanded is known as the Joule-Thompson effect. Joule-Thompson expansion
is a useful technique for lowering the temperature of the hydrocarbon gas streams of the present
invention.

[0027] The feed liquid 25 from the separator 205 is divided in two feed liquid portions

27, 29. One portion 27 is expanded through an expansion valve 241 where its temperature is
reduced to about -57°C to provide expanded feed stream 35. This expanded feed stream 35 from
the expansion valve 241 is used for cooling purposes in the heat exchanger 229 at the overhead
of de-ethanizer column 203. The feed stream 37 exiting from exchanger 229 is forwarded to the

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cryogenic heat exchanger 223 to provide cooling capacity thereat. After providing cooling
capacity feed stream 37 exits the cryogenic heat exchanger 223 as stream 39.

[0028] The other feed liquid portion 29 is expanded through an expansion valve 243 to

provide expanded feed stream 3 1 . The expanded feed stream 3 1 from expansion valve 243 is
then routed to the cryogenic heat exchanger 223 to also provide cooling capacity thereat. Feed
stream 33 exits the cryogenic heat exchanger 223 after providing its cooling capacity. Streams
33 and 39 are recombined to form demethanizer feed stream 41. Demethanizer feed stream 41 is
fed to the mid-region of the demethanizer column 201, which is below the entry location of
demethanizer feed stream 23.

[0029] The demethanizer 201 is a reboiled absorber type of column where methane is

mainly separated at the top and ethane and heavier hydrocarbons are concentrated in the bottom
of the column. The demethanizer column 201 has two side reboilers 211 and 213 and a main
thermosyphon reboiler 225 to provide enthalpy to the column.

[0030] Demethanizer overhead stream 43 is rich in methane't97^% molar), leaves

the top of the demethanizer colunm 201 at about -95° C. The demethanizeToverhead stream 43
is routed to the cryogenic heat exchanger 223 where it gives cooling capacity thereat and it is
warmed to about -9°C. The methane-rich stream 45 exiting the cryogenic heat exchanger 223 is
ftirther warmed to about 25°C at heat exchanger 22 1 . The methane-rich stream 47 from heat
exchanger 221 goes to a first compressor 234, where its temperature increases up to about 1 14°C
after compression. The compressed methane-rich stream 49 fi*om the first compressor 234 is
cooled first with cooling water (not shown) in exchanger 215 and then further cooled by
supplying heat to the main reboiler 225 of the demethanizer column 201 .

[0031] The methane-rich stream 53 from reboiler 225 is at about at 5°C. This methane-

rich stream 53 is further compressed in a second booster compressor 235 where its temperature
increases up to about 60°C after compression. The compressed methane-rich gas 55 from the
second booster compressor 235 is then cooled with cooling water (not shown) in exchanger 217.
The cooled methane-rich stream 59 from exchanger 217 is then divided into parallel streams 61,

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65. Stream 65 is cooled in exchanger 219. Stream 61 is cooled in exchanger 221. The cooled
streams 63 and 67 from respective exchanges 221 and 219 are recombined to form stream 69,
which has a temperature of approximately -7°C. Stream 69 enters a turboexpander 237.

[0032] The turboexpander 237 is a work expansion machine in which mechanical energy

is extracted from a high-pressure gas, such as stream 69. This machine expands the gas in a
substantially isoentropic manner to the pressure of the demethanizer colunan 201 . The work
from the expansion drives the booster compressor 235. After expansion, expanded stream 71
exits turboexpander 237 at a temperature of about -96°C. This cooled and expanded stream 71
provides cooling capacity to the cryogenic heat exchanger 223. After providing cooling capacity
expanded stream 73 exits cryogenic heat exchanger 223 at about -9T. The stream 73, which is
rich in methane, is further warmed to about 25°C at heat exchanger 219. The methane-rich
stream 75 exiting from heat exchanger 219 is ready for sale. A portion of methane-rich stream
75 (not shown) can be recycled to the compressor 234.

[0033] The liquid stream 81 exiting from the bottom of the demethanizer column 201 is

at about -3°C and is mainly ethane, propane and heavier hydrocarbons. The stream 81 is
expanded though an expansion valve (not shown), and its temperature is reduced to
approximately -6*'C. This expanded stream is then used for cooling purposes at exchanger 209.
At exchanger 209 the stream 81 is partially vaporized before it is fed as stream 83 to the de-
ethanizer column 203.

[0034] The de-ethanizer column 203 is a fractionation colunm with a condenser 229 and

a separator 207 arrangement at the column overhead and a thermosyphon reboiler 227 at the
column bottoms. The duty of a reboiler 227 may be provided by warm water or other convenient
heat source (not shown). The liquid propane-rich stream 101 exiting the bottom of the de-
ethanizer column 203 is mainly propane (99% molar). The propane purity of stream 101
depends on, among other things, the amounts of C^^- hydrocarbons present in the hydrocarbon
feed stream 1 . Stream 101 leaves the de-ethanizer column 203 at about 53 ''C. The gas stream
85 leaving at the top of the de-ethanizer column 203 is at a temperature of about -6 ""C. The gas
stream 85 is essentially ethane (minimum 96.5% molar). The gas stream 85 is cooled in

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exchanger 229 to yield gas stream 87. Gas stream 87 is separated into gas stream 91 and liquid
stream 89 at separator 207. Gas stream 91 is used as a cold stream for cooling duty at the heat
exchangers 231 and 233 before it is sent for storage and delivery purposes as an ethane-rich
product stream 95. Liquid stream 89 from the separator 207 is retumed to de-ethanizer column
203.

[0035] Features of the present invention are further described below in the following

examples:

EXAMPLES

[0036] Process simulations were performed with the gas feeds shown above in Table I in

order to generate the outputs required to analyze and compare the results of the different feeds
and different processes. Hysys® version. 2.1 .1 from HyProtec is a software package that was
used for process simulations. The data from an existing prior art process was used to validate the
simulations.

[0037] The following parameters were used for technical evaluation:

[0038] Ethane Recovery: Ethane recovery is the percentage of ethane product recovered

from the feed. The ethane recovery was set at 92% minimum.

[0039] Horsepower Consumption: The horsepower requirements are a measure of

investment and operating cost associated with major large rotative equipment.

[0040] Total Number of Equipments: This factor is a relative measure of operational

complexity, investment cost, operating cost and maintenance cost.

[0041] Reliability: This factor considers the probability of failure for each process

scheme evaluated. The factor is a function of the equipment availability and the system

complexity.

[0042] Operational Flexibility: This factor considers the capacity of any process scheme

to obtain same ethane recovery for lower flow rates and different feed gas composition.

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[0043] The results of the simulations for the three hydrocarbon feeds described above in

Table I are compared below in Table 11.

Table II
Process Simulations

Case:

Lean

Normal

Rich

Operating Conditions

Units

Feed Gas Pressure

kg/cm2 abs

68

68

68

Feed Gas Temperature

°C

26

26

26

De-methanizer Column

Overhead Temperature

^C

-98.4

-94.6

-84.4

De-methanizer Column

Bottom Temperature

°C

-1.7

-1.6

-0.8

De-methanizer Reboilers Total

Duty

MMkcal/h

2.27

1.83

1.55

De-ethanizer Vapor Distillate

Temperature

°C

-5.6

-5.7

-5.0

De-ethanizer Condenser Duty

MMkcal/h

0.020

0.014

0.011

De-ethanizer Reboiler Duty

MMkcal/h

0.84

1.47

1.97

Methane Gas Turboexpander

Discharge Temperature

°C

-95.0

-95.9

-86.1

Ethane Recovery

%

97.0

95.9

92.1

[0044] As shown in Table II, the percentage of ethane recovered with turboexpansion

process of the present invention is about 96% for normal gas feed, about 97% for lean gas feed
and about 92% for rich gas feed. As the feed becomes leaner, the ethane recovery increases for
the process of the present invention. This is an advantage for the turboexpander scheme of the
present invention because as the concentration of methane in the feed increases, lower
temperatures are achieved as shown in Table 11. As a consequence of these lower temperatures,
higher ethane recoveries are obtained with the inventive process.

[0045] Table III is shown below where the results of the simulations for the normal feed

case described above are compared for technical evaluation with an existing prior art plant that
uses a propane refrigeration cycle to process the same feed.

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Table III
Process Comparison

Propane Methane Stream

Refrigeration Turbo-
Scheme Process Expansion Process

Operating Conditions

unus

Feed Gas Pressure

Kg/cm^ abs

/CO

06

OO

Feed Gas Temperature

zo

zo

De-methanizer Column Overhead

Temperature

-y*T,o

De-methanizer Column Bottom

Temperature

-J

- 1 .0

De-methanizer Reboilers Total

Duty

MMkcal/h

1 oc
1.50

De-ethanizer vapor distillate

Temperature

-0

7

De-ethanizer Condenser Duty

MMkcal/h

0.67

0.014

De-ethanizer Reboiler Duty

MMkcal/h

1.17

1.47

Propane Lowest Refrigeration

Temperature

°C

-33

Not Present

Methane Gas Compressor

Discharge Pressure

kg/cm2 abs

Not present

Present

Booster Compressor Discharge

Pressure

kg/cm2 abs

Not present

Present

Methane Gas Turboexpander

Discharge Temperature

Not present

-96

[0046] As shown above in Table 3, both schemes have approximately similar operating

temperatures and duties for the demethanizer and de-ethanizer columns. One significant
difference between the processes is the source of the cooling capacity.

[0047] A performance chart is shovra below in Table 4 where both schemes are

compared for a feed gas flow rate of 80 t/h and 56 t/h, or operating at 100% and 70% of capacity,
respectively.

Table IV

Comparison at Different Plant Capacities

Propane Methane Stream

Case Refrigeration Process Turboexpansion Process

100%

70%

100%

70%

Performances

Capacity

Capacity

Capacity

Capacity

Ethane Recovery

95

95

95.9

97

Methane Gas

Compressor Power

Not Present

Not Present

2713 HP

2713 HP

Propane Compressor

Power

2150 HP

2150 HP

Not Present

Not Present

Total Compression

Power ^\

l^ryOgenicjExchanger,

2150 HP

2150 HP

2713 HP

2713 HP

1.82

1.82

1.03

1.03

UA^ J

MMkcal/h°C

MMkcal/h°C

MMkcal/h°C

MMkcal/h°C

Total'Number of

Equipments

20

20

13

13

i ,. =

[0048] As shown above in Table IV, the e thane recovery achi eved v^ith the

turboexpan sionj)rocess of the present invention is greater than th at ac hi eved with propane
refrigeration, both at 100% capacity and at 70% capacity. At the turndown operation of 70%
capacity, the ethane recovery increases for the turboexpansion method. The horse power
consumption for the turboexpansion scheme is about 25% greater than the consumption with the
refrigeration scheme. The number of equipments in the turboexpansion case is 35% lower than
that for the refrigeration process. The UA of the cryogenic heat exchanger is about 75% larger
for propane refrigeration scheme.

[0049] The percentage of ethane recovered with turboexpansion process of the present

invention is about 96% for the normal feed, about 97% for the lean feed and about 92% for the
rich feed. The ethane recovery achieved with the turboexpansion process is greater than that
achieved with propane refrigeration process, both at 100% capacity and at 70% capacity. At
turndown operation, the recovery of ethane increases for the process of the present invention.
The horse power consumption for the turboexpansion process is about 25% greater than the

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consumption of the propane refrigeration process. The number of equipments in the
turboexpansion case, however, is 35% lower than that for the refrigeration process.

[0050J Various changes to the foregoing described and shown process would now be

evident to those skilled in the art. Accordingly, the particularly disclosed scope of the invention
is set forth in the following claims.

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